1. Field of the Invention
The present invention relates to a method and apparatus for enabling cost-effective duplex communication by diplexing one of down stream signals for frequency upconversion in a hybrid fiber-radio system.
2. Description of the Related Art
Increasing demands for new mobile internet services including video and interactive services have resulted in the exhaustion of the available frequency band for wireless communications, and so triggered the development of the 4th generation wireless communication system. As the next generation communication system of the IMT-2000 (International Mobile Telecommunication), micro/millimeter-wave communication utilizing signals in the band of 3 GHz to 300 GHz is drawing attention for broadband wireless communication.
However, because of its limitation on the transmission distance and need for high-frequency sources, the micro/millimeter-wave communication system must be hybrided with existing wire systems. Low-attenuation, EMI-free optical fiber transmission is considered the most promising candidate for long-haul transport of high frequency band wireless signals. Thanks to the development of optical amplifiers and WDM wavelength division multiplexing) technology, transmission capacity is remarkably increased with optical fiber communication systems. Employing optical fiber in transmitting and amplifying micro/millimeter-wave signals is advantageous in utilizing existing core optical fiber systems and mature technologies in more economical ways. In such hybrid systems, the technology for generating optical micro/millimeter-waves is the key technology for broadband communication systems.
A main issue in hybrid fiber-radio transmission systems is signal band conversion into a carrier frequency, because up/down conversion between baseband and carrier frequency in micro/millimeter-wave systems requires expensive equipment.
There are three system options: baseband signal transmission, intermediate frequency (IF) signal feeder transmission, and optical micro/millimeter-wave transmission. A disadvantage of baseband transmission is the necessary use of highly complex outdoor base stations including full SDH/SONET compliant equipment. A disadvantage of the IF feeder system is the use of moderately complex outdoor base stations including RF up/down conversion. For high radio frequency systems, however, both the baseband transmission and IF feeder transmission methods require many local oscillators to up/down convert signal frequencies into carrier frequencies. Therefore, neither baseband signal transmission nor IF signal feeder transmission are considered cost-effective solutions for pico-cell communication.
Among the system design options for the hybrid fiber-radio system, generating and transmitting optical micro/millimeter-wave signals has many advantages. For example, the optical micro/millimeter-wave transmission system requires relatively simple outdoor base stations with compact remote antenna units. In an optical micro/millimeter-wave transmission system, signals at carrier frequencies are optically generated with light sources at a central office (CO) and transmitted through optical fibers to a remote station (RS), where the signals are simply optical to electrical converted (O/E-converted). Therefore, using a micro/millimeter-wave transmission system can reduce loads of many RSs in pico-cell communication networks. Moreover, as the high-frequency optical signals are generated at a central station, the optical micro/millimeter-wave transmission system is not only cost-effective and efficient, but also allows a centralized system to be implemented. With control functions at the central station, channel allocation, hand-over, and antenna positioning can be easily controlled, and the number of subscribers that the system can support is increased.
In addition to the advantage of easy system maintenance, the optical micro/millimeter-wave transmission system has transparency to modulation types. Since the generated optical signals are modulated by electrical baseband signals, the optical micro/millimeter-wave transmission system is flexible to any type of modulation format. With these advantages, the optical micro/millimeter-wave transmission system enables cost-effective and efficient communication and provides flexibility to radio access systems. Thus, the method of optical micro/millimeter-wave generation and transmission has been applied to mobile communications and wireless subscriber loops such as mobile LANs or broadband wireless local loops (B-WLL) or LMDS systems.
Despite its many advantages, the optical micro/millimeter-wave transmission system has problems in upstream transmission from a subscriber to a head end, because in upstream transmission, it is necessary to generate an optical micro/millimeter-wave in a remote base station, which may increase the cost of RSs. Therefore, it is difficult to apply the optical micro/millimeter-wave transmission system to duplex communication systems.